The new home I purchased was configured with a 75 gallon Rheem Water Heater. Model 21VR75N, Serial# RHLN [protected]. The first incident we experienced involved a significant amount of corrosion on top of the unit where the nipples are attached for the inlet and outlet water. Rheem, when contacted for warranty service, denied it was their problem. They deferred the issue to inproper installation. Galvanic corrosion was resident to the top element of the water heater, not the nipple couplers. They denied the claim and sidestepped the opportunity to help.
A second occurrence with this same unit just happened with a complete failure of the unit due to corrosion through the underneath side of the tank near the apex of the bottom portion where the flame heats the unit. I contacted Rheem, and they were not interested in helping.
The water heater is now 6 years and 1 week old. The water heater warranty lasts for 6 years. Given they knew of the corrosion occurring with this unit, they offered no help at all. The unit appears brand new upon initial inspection. It has been destroying itself, from the inside out, due to galvanic corrosion and materials that do not stand up to the process of reduction(Corrosion).
Rheem makes the water heaters that Home Depot sells. Those units they sell are considered problematic by most if not all plumbers that must replace water heaters for customers. Beware to the DIY!!! You will have similar issues with corrosion.